1. Field of the Invention
The present invention relates to an automatic gain control (hereinafter referred to as "AGC") device in a television receiver, particularly in a high definition television receiver for receiving high definition television broadcasting.
2. Description of the Related Art
There has been proposed a band-width compression transmission system for transmitting a high definition video signal for obtaining a more precise picture than that obtainable in the existing standard television system, by use of one channel of 27 MHz band-width of the broadcasting satellite (see Yuichi Ninomiya et al., "High Definition Television Transmitting system through Satellite One-Channel (MUSE)", ITEJ Technical Report, Vol. 7, No. 44, Technical Group on Television Equipment and Broadcast Systems (TEBS), Mar. 22, 1984).
The television system described in the above reference has a feature in that the system is disadvantageous in view of S/N in comparison with the conventional broadcasting system because of its wideband signal and therefore the system is employing a positive polarity synchronizing system in order to prevent S/N from deteriorating as much as possible. The positive polarity synchronizing system will be described by reference to FIGS. 2A and 2B showing waveforms of synchronizing signals. FIG. 2A shows waveforms of vertical synchronizing signals. FIG. 2A shows the vertical synchronizing signals which exist on the 605th and 606th ones of the 1125 scanning lines of the high definition television which provide a frame pulse point 21 once every frame. In FIG. 2A, the numerical values followed by the symbol "CK" represent transmission clock. For example, 1 CK represents a period of 1/(16.2 MHz). The reference numeral 22 represents the period of a horizontal synchronizing signal (hereinafter referred to as "HD period") existing every line, which will be described later. FIG. 2B shows the waveforms of the above-mentioned horizontal synchronizing signal which is inverted in polarity every line. Reference numeral 23 represents a horizontal synchronization reference point (hereinafter referred to as "HD" point) which exists at the 6th one of the transmission clocks represented by the reference numeral 24. Reference numeral 25 represents the level of amplitude which rises and falls within a range of from 25% to 75% as shown in the drawings.
Although not shown in the drawing, the video signal has a time compression color difference signal and a time non-compression luminance signal which exist between the HD period and the HD signal on the next line.
As described above, the television video signal of this television system is of the type of positive polarity synchronizing system in which the vertical synchronizing signal has a 100% amplitude value and the horizontal synchronizing signal has a 50% amplitude value. In a television receiver according to this television system, therefore, it is impossible to perform such a simple synchronizing separation in the direction of amplitude as in the conventional system, and it is necessary to perform the synchronizing separation in a manner so that a frame pulse point is extracted based on the correlation between two lines and then a gate for the horizontal synchronizing signal portion is opened to detect the HD point. Accordingly, a certain time is required by the time the acquisition of synchronism is attained.
As described in detail in the above reference, this television system is a band-width compression system in which one picture scene is composed of pictures of 4 fields and therefore provision of a frame memory is indispensable for demodulation of the picture signal. Accordingly, it is difficult to perform signal processing with respect to the picture signal as it is in the form of an analog signal, and therefore it is necessary to perform an analog-to-digital conversion of the picture signal. To this end, an analog-digital converter (hereinafter referred to as "A/D converter") is used. In this case, however, it is necessary to set the amplitude value of the input analog signal applied to the A/D converter accurately to a 100% level in order to perform accurate signal processing. In the case where there exist various signal sources for satellite broadcasting receivers, VTRs, video discs or the like, variations may occur in signal amplitude, and in order to cope with this problem it is necessary to provide an automatic gain control circuit (hereinafter referred to as "AGC circuit") for controlling the amplitude of the input television video signal with high accuracy.
As described above, the television video signal according to this television system requires highly accurate AGC operation. To this end, it is necessary that the amplitude of the vertical synchronizing signal or horizontal synchronizing signal as defined by referring to FIGS. 2A and 2B is detected and AGC is performed so that the detected amplitude is always kept to a fixed level. Since the television video signal according to this system is of the type of positive polarity synchronization as described above, however, it is impossible to detect the amplitude value of the synchronizing signal in a period before the synchronism is not yet achieved, for example, at the time of power turn-on, to thereby make it impossible to perform AGC in that period. On the other hand, in the case where this television system is used in popular families, a problem arises in that various signal sources may be coupled with input signal terminals so that there exists a possibility that the terminating resistance is made open or double. Taking into consideration not only the above problem but variations in the terminal resistance, it is necessary to absorb the variations of input signal amplitude of about .+-.10 dB. In the synchronization system according to this television system, however, there is a possibility that the synchronizing signal detection circuit will operate erroneously to make it impossible to perform the acquisition of synchronism. Thus, it is necessary to provide a countermeasure against this problem.